This invention relates to a method of electrodepositing a hexacyano-cobalt iron complex, which is expressed by the general formula (I) and serves as an excellent electrochromic material: EQU Fe.sub.x [Co(CN).sub.6 ].sub.y (I)
where 3.ltoreq.x.ltoreq.4 and 2.ltoreq.y.ltoreq.3.
Our Japanese patent application No. 58-36570 filled on Mar. 8, 1983 discloses that a hexacyano-cobalt iron complex of the general formula (I) serves as an electrochromic material, which is colorless in the reduced state and assumes yellow color in the oxidized state and which is high in memory effects and quick in response. According to the Japanese patent application, a film of the hexacyano-cobalt iron complex is electrodeposited on a tin dioxide electrode, for example, from an aqueous mixed solution containing 0.01 mole/liter of potassium cobalt hexacyanide K.sub.3 [Co(CN).sub.6 ] and 0.01 mole/liter of an iron(III) salt such as ferric chloride by performing electrolytic reduction at a constant current density of 0.05 mA/cm.sup.2.
However, we have encountered some problems in the electrolytic reduction process for the electrodeposition of the hexacyano-cobalt iron complex. Sometimes the electrodeposited film of the cyanocomplex is not uniform in physical properties and exhibits unevenness in the tone or tint of the color of the film. This tendency becomes significant when electrodepositing the cyano-complex as an electrochromic film for use in a relatively large-sized electrochromic display device. Besides, in industrial practice of the electrolytic reduction process the efficiency of electrodeposition is rather unstable so that the obtained electrochromic films are liable to become significantly different in the quantity of electricity. In some experiments on a large number of hexacyano-cobalt iron complex films electrodeposited under the same conditions, the dispersion of the electrodeposition efficiency was found to be as large as 40 to 100% from the measurements of the quantities of electricity of the respective films in an electrolyte solution containing 1 mole/liter of sodium perchlorate dissolved in propylene carbonate.